In recent years, there are increasing demands for portable electronic devices such as cellular phones and smartphones. Such an electronic device is equipped with a small and thin imaging unit, and the imaging unit includes a solid-state image sensor (such as a CCD image sensor or a CMOS image sensor) and an optical element such as a lens in general.
For purposes of increasing the production efficiency, an optical element equipped in an electronic device is required to have heat resistance and heat yellowing resistance sufficient for mounting the element by a reflow soldering method. Besides, since use of lead is restricted due to environmental considerations in recent years, soldering is carried out using lead-free solder, and hence, further higher heat resistance (about 270° C.) is required.
It is known that a thermoplastic resin such as polycarbonate, or a thermo- or photo-curable resin such as an acrylic-based resin or a silicone-based resin is used for the above-described optical element (see, for example, Patent Literatures 1 to 4).